The basic aim of the proposed research is to study the similarities and the differences between the stem cells of teratocarcinoma (embryonal carcinoma cells - ECC) and normal embryonic cells these tumor cells stem from, in order to further characterize the aspects of neoplasia which are common to both normal embryonic and aberant neoplastic development. For this purpose teratocarcinomas will be produced by transplantation of mouse embryos or fetal genital ridges to extrauterine sites. Retransplantable teratocarcinomas and malignant embryoid bodies derived from the ascitic conversion of teratocarcinoma as well as normal embryos will be also studied. Differentiation of ECG and normal ectodermal embryonic cells will be studied with respect to the appearance of or disappearance of certain biochemical markers; changes in the fine structural appearance of cells as visualized by a transmission or scanning electron microscope; reactivity of cells to the same type of cell injury. Formation of vessels in the tumor and the embryo will be studied as a special form of differentiation. It will be determined how the blood vessels form in embryos in utero, or in embryos explanted in vitro, in embryos transplanted to extrauterine sites as well as in teratocarcinomas and malignant embryoid bodies. Vascular supply provided by the host to the transplanted embryo, embryoid bodies or retransplantable teratocarcinomas will be studied also in order to understand this form of direct interaction between the graft bearing animal and the transplant, and how it affects the growth and differentiation of the tumor.